Custody transfer transactions involving natural gas usually are based upon measurements made by a flowmeter mounted in the gas transmission line. Because flowmeters may be affected over a period of time by the presence of foreign material in the gas streams and because they are susceptible to error due to the turbulence in gas pipelines caused by elbows and valves, there is a need for in situ calibration of flowmeters to ensure that the performance remains within the accuracy specifications for the meter. The general principle usually employed in meter proving methods involves making an independent measurement of the amount of gas flowing through the meter. Typically this independent measurement requires collecting the gas which passes through the meter and then determining the amount of gas by one of two methods: 1) determining the amount of material accumulated within a fixed volume; or 2) measuring the rate at which the accumulator volume increases. Another method sometimes used in industry is a two-step method where a transfer standard such as a turbine meter is calibrated against a primary standard and then used for extensive calibration tests. This also works with the new method presented in this disclosure wherein this method is the primary standard.
The most common method of determining the amount of gas in a fixed volume accumulator is by direct weighing. The major disadvantage associated with the application of this method is that any container that can withstand the pressure normally encountered in natural gas pipelines will be sufficiently heavy that extreme precision is required in the weighing operation. As a result, it is unlikely that an accurate method could be developed based upon direct weighing that would provide calibrations accurate to better than 1% and be mobile enough to be transported to field meter locations. The problem becomes even more difficult if high flow rates and large line sizes are involved. Direct weighing can be used in laboratory situations for large flow rates, but portable gravimetric devices do not appear to be feasible at this time.
A second method used to determine the amount of gas in a fixed volume accumulator involved measuring the pressure and temperature of the gas. Once the volume of the accumulator is determined from a calibration experiment, the amount of material present may be calculated. However, because the pressure varies essentially linearly with temperature in a fixed volume, this method is highly susceptible to errors which are introduced by temperature gradients within the tank. This method is rarely used outside of carefully controlled laboratory conditions and is of little use for portable field calibrations.
Volumetric provers are even more difficult to operate reliably than gravimetric provers for large volume flows. This is because the control of the variable volume must be such that the pressure and temperature of the gas within the meter prover remains fixed during the experiment and the rate of increase in the volume is measured accurately at the same time. Measurement of large flow rates requires precise control and measurements of rapid mechanical movement. As the total flow rate increases, the reliability of all volumetric provers deteriorates rapidly.
The proposed new method for calibration of natural gas flowmeters overcomes shortcomings of the existing methods. This novel method measures the capacitance of a structure installed within a fixed volume accumulator. It will allow for easy portability and can be used with very large flow rates. The maximum flow rate measurable is determined strictly by the volume of the fixed accumulator, which for portable devices is limited only by the limits set for the maximum length and width allowed for vehicles on the highway. So this method avoids the problems associated with other techniques while at the same time it provides for easy portability and better accuracy.
This disclosure is directed to a different type of calibration apparatus. It uses a fixed volume container, and avoids the problem of weighing the container to obtain extremely small weight variations. A two plate capacitor is positioned in the container. The two plate capacitor has the form of an elongated cylindrical capacitor. Preferably, it is formed by two cylindrical plates which are spaced apart with a gap but the gap is filled with the gas being measured. It has been determined that the gas in the gap provides a change in the dielectric of the condenser which is proportional to or a function of density, and density of course is related to the pressure in the chamber. As the pressure goes up or down, the density and hence capacitance changes with it. Moreover, it changes with density substantially independent of temperature over a substantial range.
The condenser having the form of two cylindrical plates defines a probe. In that sense, it is an elongated or wand-like shape. Conveniently, it can be a simple wand with an external coating on it formed of a conductive metal thereby defining the smaller cylindrical plate. It is positioned in a fixed chamber or container. Such containers are ordinarily elongated and cylindrical, and advantage is taken of the fact that the container is usually an elongated cylinder constructed as the storage container. The inside wall can be used as the second plate. The fixed capacity storage container thus becomes part of the measurement device i.e., one plate of the capacitor. In an alternative aspect of the device, an elongated probe can be formed of first and second plates which are positioned in the container in the fashion of a removable elongated probe. Such a probe is constructed with the first and second plates separated by the same gap. The probe can become substantially rod-like i.e., it can be relatively long and quite narrow in diameter.
Such elongated probes are used in measurement of aviation fuel in the tank of military aircraft. Such high performance aircraft may invert in flight. Probes are typically used to measure the jet fuel in the aircraft i.e., a liquid having the nature of kerosene. At high altitudes when inverted, it is not uncommon for liquid fuel to form a froth where the effective fuel value of the foam is determined by the probe in the fuel tank of the high performance aircraft. Such probes however do not provide the requisite precision accuracy that is sought in the present equipment. Such aircraft related systems have an inaccuracy of about 5% when the tank is only half filled where the accuracy does improve somewhat as the fuel in the tank is depleted. The accuracy probably is around 2% when the tank is substantially empty. Such systems however find their greatest use in foaming liquids. By contrast, the present invention is very accurate, perhaps having an error of only a fraction of one object.